Experimental Investigation of the Flow Pattern and Wall Pressure Distribution in a Silo with a...

Experimental Investigation of the Flow Pattern and Wall PressureDistribution in a Silo with a Double-Cone Insert

Michał Wójcik*, Johannes Härtl**, Jin Y. Ooi**, Michael Rotter**, Songxiong Ding***,Gisle G. Enstad***(Received: 16 December 2006; in revised form: 14 June 2007; accepted: 15 June 2007)

DOI: 10.1002/ppsc.200601120

1 Introduction

Silos with shallow hoppers are frequently used in indus-try to store large amounts of material, particularly whenthe headroom is limited. Shallow hoppers induce funnelflow, which may cause problems such as segregation andpiping. One possibility for avoiding such problems is topromote mass flow by the use of inserts in the shallowhoppers. An insert is normally placed at a certain heightabove the outlet. If correctly designed and positioned, itreduces stagnant zones in a silo with a relatively shallowhopper, leading to a flow pattern approaching mass flow.However, there is still very little knowledge of the effectof inserts on flow pattern and wall pressure, and there-fore, significant challenges remain in the design and pla-cement of such an insert. Experiments with silo inserts

have been performed, e.g., Tüzun et al. [1], Johanson etal. [2], Strusch [3], Enstad [4] and Kobielak et al. [5].However, most of these experiments were performed insmall models. Since silo phenomena are known to besensitive to scale effects, these results may not be di-rectly applicable in full-scale silos.The intention of this paper is to present a recent experi-mental investigation of the wall pressure and the flowpattern observed during the filling and emptying of afull-scale silo, with and without, inserts. The experimentswere carried out with sand, at the Dept. POSTEC in theTel-Tek Research Institute in Norway (Wójcik et al.[6]). The material used in the experiments was dry sand.

2 Experimental Setup

The tests were conducted in a cylindrical steel silo witha capacity of 35 m3, Figure 1. The cylindrical section wasca. 8 m in height and 2.5 m in diameter (aspect ratio ∼3).The axisymmetric conical hopper was 1.25 m high andwas inclined to the vertical with an angle of 44°. Theoutlet diameter was 0.1 m.The wall loads were measured with the aid of 10 pres-sure cells mounted along the walls, with 7 in the hopper(H1–H3, T4–T7) and 3 in the cylindrical part (C8–C10),Figure 1b. The cells T4–T7 were placed at the same level

296 Part. Part. Syst. Charact. 24 (2007) 296–303

* M. Sc. M. Wójcik (corresponding author), Faculty of Civiland Environmental Engineering, Gdansk University ofTechnology, 11/12 Narutowicza, 80-952 Gdansk-Wrzeszcz(Poland).E-mail: [email protected]

** M. Sc. J. Härtl, Prof. J. Y. Ooi, Prof. M. Rotter, University ofEdinburgh, Institute for Infrastructure and Environment,EH9 3JN (UK).

*** PhD S. Ding, Prof. G. G. Enstad, Tel-Tek. Dept. POSTEC,Kjoelnes Ring, N-3914, Porsgrunn (Norway).

Abstract

The aim of this paper is to present a recent experimentalinvestigation of the wall pressure distribution and flowpattern observed during the filling and emptying of afull-scale silo, with and without, inserts. The experimentswere carried out with sand, at the Dept. POSTEC in the

Tel-Tek Research Institute in Norway. The results showthat the double-cone insert had a strong influence onthe flow and pressures in the silo. A skewed flow oftenoccurred when the insert was installed.

Keywords: flow pattern, funnel flow, granular materials, insert, silo pressure

http://www.ppsc-journal.com © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Part. Part. Syst. Charact. 24 (2007) 296–303 297

© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.ppsc-journal.com

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Fig. 1: Funnel flow silo: (a) geometry, (b) pressure cell and its position on the silo walls.

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Fig. 2: Markers and their specified positions in the silo.


just below the hopper transition to investigate the pres-sure symmetry. Each cell measured both normal andtangential loads, except cells C8–C10, which measuredonly normal loads. The pressure cells with a diameter of120 mm were manufactured using the material cut fromthe silo wall. Thus, the surface of each cell had the sameroughness and curvature as the silo wall. The cells werecalibrated using dead weights and found to have linearresponses. The load cell stiffness in the normal directionwas measured to be ca. 660 N/mm. The pressure signalswere recorded using a PC with a frequency of ∼500 Hz.The fill material used was dry sand with a loose pouringbulk density of 1370 kg/m3, a particle size range 0.6–2 mm,angle of repose of ∼36° and a wall friction angle of 22°.The silo was filled and emptied symmetrically. Con-centric filling from a fixed height (2.5 m below the siloroof) was chosen with a filling rate of ca. 250 kg/min.The total volume of the material was ca. 26 m3. The silowas discharged under gravity. The emptying rate was450–500 kg/min. The outflow was controlled with the aidof a pneumatic valve. It was situated 30 cm below theoutlet using a steel tube, in order to have a minimum in-fluence on flow eccentricity during the opening process.During the emptying process, the surface profiles of thesand were measured every 10 min from the top of thesilo. The nine point profile measurements were used toevaluate the repose angle and the discharge rate.To investigate the flow pattern, small markers with adiameter of 30 mm were placed at fixed positions in thesilo during filling (7 levels with 9 markers and 2 levelswith 5 markers), using a carefully installed system of lo-cating tubes on two orthogonal diametral planes, Figure2. One tube was placed at the center of the silo, 4 nearthe silo wall and 4 half way between these points. Dur-ing filling, when the lower end of the tube was coveredwith sand, the marker was dropped through the pipe.The tube was then gradually pulled up to the next higher

level. This procedure was repeated until the silo wasfilled completely. During discharge, numbered markerspassing through the outlet were detected, and the resi-dence time for each marker was noted. The flow profileduring discharge was then deduced from the residencetime measurements for the markers.Flow tests were carried out in the silo both without aninsert and with a 1.62 m high stainless steel double-coneinsert, Figure 3. The diameter of the insert was about25% of the hopper width as recommended by Enstad,based on experience from small scale tests [4]. With awall friction angle of 22° and an angle of repose of 36°representing the effective angle of internal friction, thecritical inclination for mass flow in a conical hopper be-comes 20°, with the usual safety margin of 4°. Figure 3shows the insert turned upside down, as the intended or-ientation was to have the shorter 30° cone pointingdownwards, and the taller 15° cone pointing upwards. Inthe intended orientation, the difference between thehopper inclination of 44° and the inclination of thelower insert cone of 30° is 14°, which is well below thecritical limit of 20° for mass flow. Small-scale tests hadindicated previously that a smaller inclination of theupper cone presented an advantage, and therefore, a 15°cone angle was chosen. However, ideal mass flow wasnot obtained with this insert, and thus, the upside downorientation was also investigated. The insert was placedin the hopper part with the aid of 3 steel fins. Significantcare was taken to situate the insert axisymmetrically. Atotal of 5 vertical positions were investigated and severaltests were performed for each position, as reported byWojcik et al [6]. In this paper, only some of the resultsare presented. These consist mainly of results of the silowithout an insert, and with an insert centrally mountedupside down with its top at the outlet level, as shown inFigure 3.


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Fig. 3: Double-cone insert placed in the hopper.


3 Results

3.1 Silo without Insert

The results show that without an insert, the silo exhib-ited typical funnel flow. Figure 4a presents an exampleof solid surface profiles during emptying and Figure 4bshows the residence time for the markers seeded in thetwo perpendicular planes. The residence times in Figure4b show that the markers at the central axis were firstlydischarged rapidly (within several minutes), followedthen by the markers from the top layers. The markers lo-cated away from the internal flow channel in the hopperand near the transition came out last. These observa-tions suggest that from the beginning of discharge, a nar-row flow channel was created in the center above theoutlet, mobilizing the sand from the top surface to slidedown on the surface towards the center. The material lo-cated adjacent to the wall remained stagnant until it wasexposed at the top.It should be noted that even though the silo filling andemptying were axisymmetric, the flow profile was notexactly symmetric. The results suggest also that thecentral rapidly flowing channel may be surrounded byanother slower flowing zone. The markers that arecaught up in this slower downward flow zone would bedelayed from being engaged in the surface flow towardsthe center. That could be the reason why the markers


a) b)

Fig. 4: Silo without an insert: (a) solid surface profiles, (b) residence times for markers at different levels.

Fig. 5: Development of the normal pressure on the silo walls with-out an insert.


located next to the wall came out faster than those inbetween the wall and the central axis. This phenom-enon appears at Levels V–VII in the upper part ofthe bin.Figure 5 presents an example of the normal wall pres-sure evolution with time during filling and emptying.Each curve follows Janssen’s trend for filling. At thecommencement of flow, an increase in the wall pressuresat certain locations (mostly the areas close to the transi-tion: H3, T4, C8) was observed. As the silo is being dis-charged, the pressure decreases to the zero value. Thelargest stress appears in the upper part of the hopper.Figure 6 shows the mean pressure distribution obtainedfrom several tests without an insert. The results werefound to be highly repeatable. At the end of filling, themaximum coefficient of variation for normal pressurewas 4 % for cells H1–H3, 9.5 % for cells C8–10 and6.6 % for cells T4–T7, whereas at the beginning of theemptying process it was 2.9, 11, and 5.5 % respectively.The pressure results along the transition indicate signifi-cant pressure variations along the circumference at theend of filling and at the beginning of emptying. The dif-ference between the highest and the lowest values at thetransition is up to 30 %. The fact that such asymmetrycan occur at different heights of the silo is of particularimportance for the structural design of silos, since it in-troduces high membrane forces into the steel silo wall,and may cause a buckling failure, as discussed byRotter [7].

3.2 Silo with Double-Cone Insert

With the double-cone insert installed, the flow channelwidened considerably but typical mass flow was neverachieved in any of the 5 positions. A possible reason forthis could be that the insert was too small comparedwith the diameter of the silo. Figure 7 shows an exampleof the surface profile and the markers’ residence timesin the silo with a double-cone insert in its upside downposition. The markers in the center and in the middlebetween the wall and the center came out first in a“mass-flow” order but markers located adjacent to thewalls came out top down, level by level implying thatthey remained stagnant until they were caught up withthe surface flow. It was also observed that at the begin-ning of discharge, the upper part of the sand also flowedslowly down along the wall, which was not the case inthe silo without an insert. During the discharge process,the diameter of the flowing channel observed from thetop became smaller and smaller until the surface profilereached the shape of the dip and the material started toslide down on the surface.

The presence of the double cone insert is noted to havea significant influence on the eccentricity of flow, as ob-served from the surface profiles and the residence timemeasurements. The most uniform (axisymmetric) dis-charge of the silo was obtained with the insert in its up-side down position, and its lower end at the center ofthe outlet. The other 4 insert positions investigated, i.e.,locations higher above the outlet and in upright posi-tion, very often caused the flow to be very nonuniform.Complete skew flow was achieved with the insert placedin its upright position with its lower end just 5 cm off-center. This phenomenon is shown in Figures 8a and 8b,where the solid surface profile and residence times formarkers at different levels are presented.


a)

b)

Fig. 6: Mean value of normal and shear stress (a) along the height,and (b) at the transition of the silo without an insert.


During emptying of the silo with an insert, the wall pres-sure showed a “non-smooth” character, Figure 9, whichindicates a much thinner stagnant zone as comparedwith results obtained without an insert (see Figure 5).

The results with an insert differ from one test to anothermore than those without an insert. At the end of filling,the maximum coefficient of variation was 7% for cellsH1–H3, 8.8 % for C8–C10 and 12.5 % for T4–T7. At the


a) b)

Fig. 7: Silo with an insert: (a) solid surface profiles, (b) residence times for markers at different levels.

a) b)

Fig. 8: Silo with insert located 5 cm off-center: (a) solid surface profiles, (b) residence times for markers at different levels.


beginning of emptying it was 12.2, 8.7, and 12.2 %, re-spectively.Figure 10 presents the mean pressure distribution ob-tained from several tests with an insert. At the com-mencement of discharge, the pressure is seen to increasearound the transition and in the bin. In the lower part ofthe hopper the pressure actually decreased. The maxi-mum pressure was seen at the transition and the in-crease of the normal pressure was up to 25 % and shearpressure up to 90 %. The increase of pressure at the on-set of emptying was more significant in the silo with aninsert than that without an insert.As compared with the tests without an insert, the pre-sence of an insert in such a configuration had a stronginfluence on the pressures in the upper part of the hop-per (significant increase of normal and shear pressure)as well as in the cylindrical part, especially at 1.8 mabove the transition level of cell C9. The pressure in thelower part of the hopper with an insert was similar tothe silo without an insert, but other positions of the in-

sert that were investigated, e.g., having the insert locatedhigher above the outlet, caused the loads to decrease.

4 Conclusions

The results show that the presence and position of an in-sert influence the flow pattern and wall pressures signifi-cantly during the filling and emptying of a full-scale silo.The flow channel is considerably widened with the in-sert, but mass flow is not obtained. In most cases, thepressures on the walls are higher with an insert thanwithout an insert. Further experimental and theoreticalinvestigations with the inserts have continued, includingexamination of other types of insert such as cone-in-cone, cone-in-cone-in-cone and inverted cone (Härtl etal. [8]) and these will be reported at a later stage.


Fig. 9: Development of the normal wall pressure in the silo with adouble-cone insert.

a)

b)

Fig. 10: Mean value of the normal and shear stress along (a) theheight, and (b) at the transition of the silo with an insert.


5 References

[1] U. Tüzun, R. M. Neddermann, Flow of Granular Materi-als Around Obstacles. Bulk Solids Handling 1983, 3,507–517.

[2] J. R. Johanson, W. K. Kleysteuber, Flow Corrective In-serts in Bins. Chem. Eng. Prog. 1996, 62, 79–83.

[3] J. Strusch, Wandnormalspannungen in einem Silo mitEinbau und Kräfte auf Einbauten, Ph.D. Thesis, Tech-nische Universität Braunschweig 1996.

[4] G. G. Enstad, Use of Inverted Cones and Double Conesas Inserts for Obtaining Mass Flow. POSTEC-Newslett.1999, 17, 15–16.

[5] S. Kobielak, A. Zamorski, Redistribution of Grain Pres-sure in Silos with Inserts. 3rd Israeli Conf. on ConveyingHandling Particulate Solids, Dead Sea, Israel, May 2000.

[6] M. Wojcik, J. Härtl, J. Ooi, M. Rotter, S. Ding, G. G. En-stad, Experimental Investigations of Stresses and Flowin a Full Scale Silo, with and without, a Double Cone In-sert. Tel-Tek-Report No. 4104010, University Porsgrunn,Norway 2004.

[7] J. M. Rotter, J. Y. Ooi, Z. Zhong, Critical Pressure Con-ditions in Silos. Proc. 5th Int. Conf. on Conveying Hand-ling Part. Solids, Sorrento, Italy, August 2006.

[8] J. Härtl, M. Wojcik, J. Ooi, M. Rotter, S. Ding, G. G. En-stad, The Influence of a Cone-in-Cone Insert on FlowPattern and Wall Pressure in a Full-Scale Silo. Tel-Tek-Report No. 4104011, Porsgrunn, Norway 2006.


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